The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for portable MRI using an inhomogeneous magnetic field for spatial encoding.
A conventional MRI scanner uses several different magnetic fields to produce an image. One field is a static, highly uniform magnetic field that is used to polarize the nuclear magnetization and in which a free-induction decay (“FID”) signal is read-out. Another field is a radio frequency (“RF”) pulsed field to initiate the FID. Also, one or more gradient fields are used to encode the spatial location from which the FID originates, thereby spatially encoding the resulting image. The gradient fields are spatially changing (e.g., linear with position) gradient fields that modulate the spin phase as a function of position. As a result of the modulate spin phase, the location of the signal is encoded as the Fourier transform of the acquired signal. Sometimes a non-homogeneous pre-polarization field is used to boost the initial magnetization, which is then read-out in a uniform, lower strength field.
Much of the size and complexity of an MRI system derives from the fact that typical clinical MRI systems require a very homogeneous static magnetic field and very high-power linear gradient fields. Thus, current MRI systems are limited to hospital settings due to the weight and fragility of highly homogeneous superconducting or permanent magnets. An additional burden is the need for hundreds of amps of current to power the gradient fields. To make the MRI system portable, a new type of MR encoding scheme is needed. It has not been sufficient to simply “shrink down” current designs.
A portable MR system has the potential to quickly detect brain injury at the site of injury. For example hemorrhage detection is critical for both stroke patients and traumatic brain injury victims. In stroke, rapid distinction between a hemorrhagic and non-hemorrhagic event could allow administration of a clot-busting drug such as tPA (tissue plasminogen activator) in an ambulance prior to transportation to the hospital, perhaps advancing this time-sensitive treatment by up to an hour. Subdural hemorrhage (or hematoma) is a form of traumatic brain injury, in which blood gathers between the dura and arachnoid mater (in meningeal layer) and is likely to be visualized on course resolution (e.g. 5 mm) T1 images. Rapid diagnostics of the hemorrhage in the field could substantially accelerate treatment by adverting a “wait and see” approach. After brain surgery, some patients develop a hemorrhage that must be treated immediately. A bedside MRI in a neuro-ICU would allow frequent checks for the development of such hemorrhaging.
Others have disclosed portable nuclear magnetic resonance spectrometers; however, even these systems still make use of conventional encoding schemes that require gradients.
It would therefore be desirable to provide a portable MRI system that does not require the use of magnetic field gradient coils to provide spatial encoding of magnetic resonance signals.